Beam-focusing for mm-wave TWTs

Summary form only given. An experimental device has been designed to demonstrate small-tunnel beam focusing. A PPM field inside a circuit section having a 0.030-inch diameter beam tunnel is expected to provide excellent transmission for beams within a range around 18 kV and 0.24 A. Transmission contour plots from DEMEOS predictions are compared with those from experimental data. The beams are formed utilizing a tetrode-type electron gun. Such beams can exhibit dramatically reduced transverse temperatures for properly chosen grid voltages, a feature that is best illustrated by the shape of a gun "hairpin" curve. For this tetrode gun, the theoretical hairpin curve is compared with measurements and the narrow nature of the proper operating voltages is discussed. The ability to join tight-tolerance parts into highly concentric assemblies is expected to be verified using a novel body-current diagnostic. This diagnostic is comprised of an electrically isolated tunnel segment that is sectioned in quarters to measure body current azimuthally about the tunnel. An estimate of assembly non-concentricity is compared with measurements of beam non-concentricity. The device utilizes a post-type conduction cooled single-stage depressed collector to allow full-duty operation. Experimental defocusing experiments at high duty will provide allowable dissipation levels for the inner grid and the tunnel wall. These data are expected to suggest designs that can be useful for PPM-focused, coupled-cavity or folded-waveguide TWTs at mm-wave frequencies.